The Role of Growth Substances in Vegetative 

 Development as Exemplified in Tissue Cultures 



PHILIP R. WHITE 



GROWTH substances and their manipulation have played a con- 

 siderable role in the development of plant tissue cultures as such, 

 and in the control of their development and differentiation. Indeed 

 Haberlandt's theory of wound hormones, leptohormones, and other 

 growth substances arose originally out of his attempts to explain the 

 difficulties encountered by himself and others in their attempts to 

 establish plant tissue cultures. 



Vegetative tissues of most if not all plants, when excised and placed 

 on a moist substratum whether nutritive or not, will undergo a consider- 

 able initial enlargement due chiefly to hyperhydric increase in size of 

 the surface cells. There is in general very little polarity to this enlarge- 

 ment if the fragment is small and if it is taken from a relatively un- 

 differentiated region such as the pith, the cortical parenchyma, the 

 interior of tubers or fleshy roots, etc. Larger or better differentiated 

 masses may show some tendency to localized or even tubercular en- 

 largement, but this is exceptional in the initial phases. This initial 

 expansion is largely a physical response due to alteration in stresses, 

 in the respiratory processes of the traumatized and exposed surfaces, 

 in osmotic patterns, and so on. It has nothing whatever to do with 

 normal proliferative growth. Indeed, tissues of some plants, such as 

 those from mature tubers of Helia7rthiis tiiberosa, do not ordinarily show 

 even this initial enlargement. These unresponsive tissues are now 

 thought to be almost devoid of residual auxins, and the more usual 

 initial enlargement is believed to be due, in part, to the activity of 

 auxins which are present in most excised tissues. The ratio of available 



